The invention relates to an arrangement for focusing the image of an object onto an image plane, in which the image is derived from an objective lens system, adapted to be adjusted by a setting device, employing two optical devices which are offset, with respect to one another, transversely to the optical axis, and which is provided with a device which analyzes the two resulting auxiliary images, in the form of electrical signals, and serves to control the adjusting device.
A general arrangement of this type is basically described in the magazine "Popular Photography", volume 78, No. 1, 1976, pages 92, 94 and 134, including the illustration on page 92 thereof. In the described arrangement, the two auxiliary images, one of which is obtained by means of a rotatable mirror coupled to the range adjusting mechanism of the objective lens system, are each reflected upon a photo-detector. When the rotatable mirror has reached the position in which congruent auxiliary images are obtained over both optical devices, the objective distance from the image plane is also set to a value at which the resultant image of the object, derived over the objective lens system, is focused onto the image plane.
Another known device of this type, in which the two auxiliary images are obtained over two subsidiary optical openings or pupils which supply different subsidiary areas of the entrance pupil of the objective lens system, is described in a magazine: "Electronic Focusing for Cameras of the Future", published by Ernest Leitz GmbH, Wetzlar, List 100-22, IX/76/LX/Bm in 1976.
The disadvantage of the known arrangements is that each of the photo-detectors produces an electrical signal which is obtained by integration over the brightness values of the overall image content. The congruency of the two auxiliary images produced by comparing two such signals cannot be established with a high degree of accuracy.